Conventionally, as a type of oil seal, an oil seal for reciprocation motion is known that includes a resin backup ring, made of fluorocarbon resin and the like, to ensure durability of a seal lip. The backup ring is disposed such as to support a seal surface of the seal lip. Among oil seals for reciprocation motion such as this, an oil seal for reciprocation motion is known in which an annular groove that can hold a lubricating agent is provided on an inner peripheral surface of the backup ring that extends in parallel with a reciprocating shaft in a setting state in which the oil seal for reciprocation motion is mounted in a sealed location. As a result, when pressure increases on the sealed side, lubrication can be ensured by the lubricating agent filling the annular groove, even when the inner peripheral surface of the backup ring is in contact with the reciprocating shaft over its entire surface. Increase in frictional force and increase in abrasion can be prevented (refer to, for example, Patent Literature 1)
Patent Literature 1: Japanese Unexamined Utility Model Application Publication No. Showa 60-110767 (FIG. 1 and FIG. 3)
However, conventional oil seals for reciprocation motion have a problem in that the oil seals cannot handle demands for high performance made in recent years.
For example, in a conventional oil seal for reciprocation motion, the inner peripheral surface of the resin backup ring is disposed parallel with an outer peripheral surface of the reciprocating shaft. Therefore, when the oil seal is used under high pressure, such as pressure of 2 MPa to 9 MPa on the sealed side, the inner peripheral surface of the backup ring comes into contact with the reciprocating shaft over its entire surface. The annular groove is crushed by the pressure and can no longer hold the lubricating agent.
As a result, abrasion and deformation of the inner peripheral surface of the backup ring advances. A stable sealing performance cannot be maintained by the overall oil seal over a long period of time. Durability deteriorates, and life is shortened.
Therefore, an oil seal for reciprocation motion is required that can achieve high performance with certainty, by preventing the inner peripheral surface of the backup ring from coming into contact with the reciprocating shaft over its entire surface, even when used under high pressure.